Update ahat version number to 1.7.3

Update README.txt to include added bitmap related changes

Bug:  347355061

Test: atest ahat-tests

Test: open heap dump with bitmaps included and verify
      1) bitmaps can be displayed
      2) duplicated bitmaps are listed and ordered by size

Change-Id: Icc4b2f07e211905436f7de2cf1f7b3b2b9e7e443