Rename InstallSource to InstallReason for metrics(part 3).

Doc: https://docs.google.com/document/d/1h2aSTCJJ815ORzEPzkr7CJIWiQYnBYUOn_lQqDInPJ0

apps::mojom::InstallSource is renamed to InstallReason, so that we can
add a new InstallSource to define where the app is installed, e.g. from
play store, chrome web store, etc.

This is the part 3 to rename InstallSource to InstallReason in
AppPlatformMetrics to align with apps::mojom::InstallReason, and
address the related build issue, modify the unit tests.

There will be follow up CLs to change other related function, variable
names[TODO]:
1. Modify crosapi to rename InstallSource to InstallReason.
2. Modify other code, to rename the related functions, variables from
InstallSource to InstallReason.

This CL is pure renaming/refactor, no function change.

BUG=1250971

Change-Id: I899ce9ecc5f0884163aaf8c974203c1be4bf00a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3192877
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#927151}