[online_image] Remove stored RAMAllocator member from DownloadBuffer

RAMAllocator with default flags is stateless — it's just a dispatch
wrapper over heap_caps_malloc/realloc/free. There's no need to store
it as a class member. Use stack-local instances at each call site
instead, matching the pattern used in audio_transfer_buffer and
ring_buffer.

Co-Authored-By: J. Nick Koston <nick@koston.org>

esphome/components/online_image/download_buffer.cpp

@@ -7,7 +7,8 @@ namespace esphome::online_image {
 static const char *const TAG = "online_image.download_buffer";
 
 DownloadBuffer::DownloadBuffer(size_t size) : size_(size) {
-  this->buffer_ = this->allocator_.allocate(size);
+  RAMAllocator<uint8_t> allocator;
+  this->buffer_ = allocator.allocate(size);
   this->reset();
   if (!this->buffer_) {
     ESP_LOGE(TAG, "Initial allocation of download buffer failed!");
@@ -38,15 +39,16 @@ size_t DownloadBuffer::resize(size_t size) {
     // Avoid useless reallocations; if the buffer is big enough, don't reallocate.
     return this->size_;
   }
-  this->allocator_.deallocate(this->buffer_, this->size_);
-  this->buffer_ = this->allocator_.allocate(size);
+  RAMAllocator<uint8_t> allocator;
+  allocator.deallocate(this->buffer_, this->size_);
+  this->buffer_ = allocator.allocate(size);
   this->reset();
   if (this->buffer_) {
     this->size_ = size;
     return size;
   } else {
     ESP_LOGE(TAG, "allocation of %zu bytes failed. Biggest block in heap: %zu Bytes", size,
-             this->allocator_.get_max_free_block_size());
+             allocator.get_max_free_block_size());
     this->size_ = 0;
     return 0;
   }

esphome/components/online_image/download_buffer.h

@@ -15,7 +15,10 @@ namespace esphome::online_image {
 class DownloadBuffer {
  public:
   DownloadBuffer(size_t size);
-  ~DownloadBuffer() { this->allocator_.deallocate(this->buffer_, this->size_); }
+  ~DownloadBuffer() {
+    RAMAllocator<uint8_t> allocator;
+    allocator.deallocate(this->buffer_, this->size_);
+  }
 
   uint8_t *data(size_t offset = 0);
   uint8_t *append() { return this->data(this->unread_); }
@@ -34,7 +37,6 @@ class DownloadBuffer {
   size_t resize(size_t size);
 
  protected:
-  RAMAllocator<uint8_t> allocator_{};
   uint8_t *buffer_;
   size_t size_;
   /** Total number of downloaded bytes not yet read. */