Fix render thread priority for allocateBuffers

Increase the prio for the thread allocateBuffer runs on
 so we don't cause the main RT prio thread wait as long
 during frame draws.

Bug: 399701430
Test: app launch test
Flag: com.android.graphics.libgui.flags.allocate_buffer_priority

Change-Id: Ic56697866f1f58c6c5575909da9e1ecd95a3cb06