Fix metalava api baseline update command

metalava is run inside sbox with a modified $PWD, so putting $PWD in
the output message results in an incorrect path.  It was also always
incorrect when the output directory was an absolute path.  Add a
cd $ANDROID_BUILD_TOP to the command line and use relative paths
instead.

Bug: 185516277
Test: m out/soong/.intermediates/frameworks/base/system-api-stubs-docs-non-updatable/android_common/metalava/api_lint.timestamp with lint error
Change-Id: Iefe133cea4c3a604ecd2b0ea20f4ba14ae13b425