SF: In dumpsys add head row for "Allocated buffers" table

Now the table looks like this:

    Handle |        Size |     W (Stride) x H | Layers |   Format |      Usage | Requestor
0xead7d500 | 8100.00 KiB | 1920 (1920) x 1080 |      1 |       2B | 0x20000b00 | BootAnimation#0
0xeaf17000 | 8100.00 KiB | 1920 (1920) x 1080 |      1 |        1 | 0x    1a00 | FramebufferSurface
0xeaf17100 | 8100.00 KiB | 1920 (1920) x 1080 |      1 |        1 | 0x    1a00 | FramebufferSurface

Test: adb shell dumpsys SurfaceFlinger
Change-Id: I5f8c77375818e5232d98b8631a39a0a0545d9224